There are two objectives. First, to complete a program which will allow, by computer evaluation, a determination of the distribution of magnetic dust in the lung. Subjects who have acquired magnetic dust in the lung either by voluntary inhalation of trace amounts or from occupational exposure will be measured magnetically. The measurements will be used in a computer program to determine the distribution of dust in the lung. This has applications to deposition patterns, clearance patterns, and perhaps early physiological changes because of the magnetic relaxation phenomenon. The second objective is to evaluate the results of the study of the Quebec asbestos miners and millers. Good correlation has now been found between the magnetic measurements of these workers and the inhaled exposure index which uses the midget impinger. Applications of the magnetic monitoring technique will be assessed; in particular, will simple magnetic monitoring of asbestos miners and millers be of direct or long-range use? Are further measurements of asbestos workers warranted?